Most garages, expected to house gas-breathing cars, have substantial air vents down low, at least along one side wall.
Many do not have finished or insulated walls and overhead.
Air leaks around the top, sides, and bottom of many garage doors can be substantial.
The garage doors and windows (if any) often have leaks or very low (single-pane) R-values.
The Garage door panels should be insulated, but the "weather stripping" is also very important. If they say "it can't be done", find another installer that WILL do it.
Remember, heat rises and cold sinks, even after you have stopped all the "drafts".
See SOC/GID-Meter and CAN-Do Info
2011 LEAF, sold in 2015
2010 Prius, 2014 silver Tesla S
Nissan EVSE, mod to 240/120v 16A
Solar PV: 33 x 225W -> 7 kW max AC
To Sell: X-treme 5000Li EV motorcycle